It depends on car make/model but a really doubt it is a ground. If it was a ground issue it would not of suddenly appeared after many years and would be likely intermittent too. You could spend a lot of time chasing gremlins looking for ground issues. You mentioned it is a older car. Swap out alternator with a rebuilt one.

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--------------------------------------Ham since 1969.... Old School 20wpm REAL Extra Class..

It is not going to cost you anything to check the ground contacts first, before you run out and buy a rebuilt alternator. We are running two Ford vehicles that are over ten years old and do not have a whine problem at this point.

If that doesn't fix it, then the alternator is probably going to have to be replaced.

The grounds you're looking for are not that many, and not hard to locate.

The way I do it is to run the engine and turn on some loads, like the heater fan on high with the headlights and rear window defroster on. Then I use a digital voltmeter with one probe on the negative battery terminal and start touching the other probe to different parts of the engine and chassis. With good bonding, you should never see any voltage difference at all, maybe a tenth of a volt at most. When a ground is compromised, you may see upwards of a volt or more. This is usually accompanied by other symptoms like lights that brighten and dim, radio memories that reset, blowing bulbs, and other untold anomalies.

If it was a ground issue it would not of suddenly appeared after many years and would be likely intermittent too.

When you drive cars old enough to vote, all sorts of strange things happen. I've troubleshot it twice on my own cars and dozens of times when I worked at a 2-way radio shop. On my cars, both times it was due to negative/ground wires that corroded inside the insulation where you couldn't see it, leaving only a few strands to carry the current. No sudden failures, and not intermittent. Just a gradual worsening of symptoms.

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Swap out alternator with a rebuilt one.

Years ago when a rebuilt alternator was $75 that might be good advice. I looked up a 2005 rebuilt Focus alternator at NAPA and they're going for about $175. That's an expensive "try it and see". It would only take a few minutes to check the grounds with a voltmeter. That would include the radio grounds too. If the negative power lead for the radio is compromised it could be getting return current through the coax, which can also cause wonky behavior.

Excellent advice and "how to" information from K5LXP on checking ground connections. It could be that replacing an alternator has worked in some cases because the ground connection was improved during the swap out.

Auto Zone sells alternators for most vehicles, including Fords, with a "limited lifetime warranty" starting at $129.00, plus a $9.95 core charge and tax. Unfortunately, a lot of auto parts suppliers will not let you return an electrical part if the problem is found to be somewhere else. That includes Auto Zone. Check those grounds first!

Check the ground strap(s) between the engine and the firewall/fender. That is the first place that I would look. If you can't find one, try putting a heavy wire from an engine bolt to the bolt on the car where the negative battery terminal is grounded. If the whine disappears, you know you've got to look for and replace that/those ground strap(s) that are going bad.

Well, lots of things just suddenly go bad, and cause a ground loop to appear out of nowhere. I will agree that older alternator diode banks aren't as good as the newer ones, but if it is a bad alternator diode, the whine will be very raspy sounding compared to one caused by a ground loop. Easy to check with an O-scope.

If you lose the fuse in the negative lead (for whatever reason), and draw power through the antenna connection of chassis of the transceiver, bingo! Even losing the battery's chassis ground, and relying on the engine ground can cause the same issue.

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